Electroluminescent-ferroelectric-photoconductive display device



Oct. 15, 1963 J. A. AsARs 3,107,314

ELECTROLUMINESCENT-FERROELECTRIC-PHOTOCONDUCTIVE DISPLAY DEVICE FiledAug. 23, 1961 be Db Fig. I

euoFF ecoN edled2- ecoFF ebON eboFF WITNESSES 2 INVENTOR Q- Juris A.Asors 3,197,314 ELECTROLUMINESQENT-FERROELEQTRF- PHOTOCONDUiZ'EEVEBZSELAY BEVECE Enris A. Asars, Monroeviile, Pa, assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa., a corporation ofiennsyivania Filed Aug. 23, 196i, Ser. No. 133,366 (lit-rims. (Cl.315-151) The present invention relates to electrical-to-light transducerapparatus, and more particularly toelectroluminescent-ferroelectric-photoconductive display apparatushaving two stable states.

In most electrical-to-light information display transducer apparatus,which are compatible with high-information-capacity solid state displaydevices, the screen of the display apparatus receives electricalinformation from a digital data processing computer. As a result, thescreen must be able to accept electrical information at a rate which iscompatible with the computer, and in most cases only a bistable (ON-OFF)mode of operation is required. Because the computer must perform otherfunctions in addition to providing electrical information to tie displayscreen, an infinite display storage time and both selective and totalerasure of old information is very desirable. Also, it is desirable tohave a screen display which can be used as a memory source with read-outcapabilities. The electroluminescent-ferroelectric (ELF) display screen,as known in the prior art, possesses the fast writing or access speedand selective erasure, as well as read-out capabilities; however, itlacks the infinite display storage device and the whole display cannotbe non-selectively erased in a short period of time. On the other hand,the electroluminescent-photoconductor display screen, known in the priorart, has infinite display storage time and can be erased on a totaldisplay basis; however, it is not compatible with computer writingspeeds and selective erasure of old information is not readilyobtainable. By combining the two display methods, apparatus, asdisclosed herein, may be provided with fast writing speed and infinitestorage time.

It is, therefore, an object of the present invention to provide a newand improved electroluminescent-ferroelectric-photoconductor displayapparatus having fast writing speed and infinite storage time.

Broadly, the present invention provides bistableelectroluminescent-ferroelectric-photoconductor display apparatus inwhich a light output is supplied from an electroluminescent cell inresponse to electrical information applied as a control potential to anelectroluminescentferroelectric circuit. A photo-conductive element isprovided to be responsive to the light output as a feedback control tomaintain the light output at the then present stable state.

These and other objects will become more apparent when considered inview of the following specification and drawings, in which:

FIGURE 1 is a schematic diagram of the circuitry embodied in the presentinvention; and

FIG. 2 is a waveform diagram used to aid in the explanation of theoperation of the present invention.

Referring to FIG. 1, an electroluminescent-ferroelectrio bridge circuitand excitation sources are shown including the nonlinear fcrroelectriocapacitors P and F the electroluminescent cell EL, the alternatingexcitation sources e and e and the D.C. bias source E The waveforms forthe excitation sources are also shown in FIG. 1. Letting 11;, be theinstantaneous voltage across the electroluminescent capacitor EL, theA.C. component of this voltage V is the light-producing excitation. Theaverage or 11C. component of V is V the control potential. Due to thenonlinearity of the ferroelectric caadd pacitors and the dissymmetry ofthe excitation and bias sources, V is a function of V Therefore, theamount of light emitted by the electroluminescent cell EL is dependentupon the control potential V Referring also to the Waveform diagramshown in FIG. 2, for bistable (ON-OFF) operation direct controlpotentials must be supplied to the point f in FIG. 1 in order toestablish the ON-OFF conditions. The potential e is defined as the ONcontrol potential, and the potential ecoFF is defined as the OFF controlpotential, as shown in FIG. 2. The potentials e and ecoFF areestablished at the point 0 shown in FIG. 1. These potentials are storedon the storage capacitor C and transferred through the isolationresistor R to the junction point 1 between the ferroelectric capacitorsF and F It is assumed in the present example that the ON potential e isgreater than the OFF potential e Point g is supplied with a DC.potential which is equal to or greater than the ON control potential eand point It is supplied with a DC. potential which is less than orequal to the OFF potential e A photoconductive resistor R is connectedto the resistor R, and the resistor R at the common point c. The otherend of the resistor R is connected to the point h. While theelectroluminescent cell BL is in the OFF state, the resistance of thephotoconductive resistor R is greater (several times) than theresistance of the resistor R and so the D.C. level at points 0 and fdrifts in the direction of the OFF state to maintain stability in thatstate. However, when the electroluminescent cell EL is in the ON state,the light feedback from the electroluminescent cell EL acting upon thephotoconductive resistor R reduces the resistance of R below that of theresistor R and the DC. level at points 0 and then drifts in thedirection of the ON level. i

In order to provide bistable operation incoming information is appliedat points a and b. As shown in 1G. 2, potentials e and e are supplied atthe points a and b, respectively, to provide the ON state, while thepotentials eaoFF and c are supplied in order to provide the OFF state.nonconducting to maintain the stored charge on the capacitor C withinthe potential limits of e to eaOFF for point a, e to eboFF for point 1;,and e to ecOFF for point 0.

Assuming that the electroluminescent cell is in its ON state, thecontrol potential c is established through the potentials e and e Whennew information is received for the particular element, it is stored onthe indirect storage capacitor C, by applying a switching burst e seeFIG. 2, to the point 61 common to the capacitor C, in coincidence withthe proper potentials eaOFF and e at the points a and b, respectively.The clamping action of the diodes D and D then may readjust thepotential on the storage capacitor C to the new control potential e andthus turning the electroluminescent cell EL to its OFF position. Withthe cell EL in its OFF position, no light feedback is provided to thephotoconductive resistor R so the resistor R goes to its high impedancestate being several times larger than the resistor R The effect then isto maintain the control points c and f at the OFF condition by havingthe control potential drift in the direction of the potential 2 or thatat point h. Now with the electroluminescent cell EL in its OFF state andnew information e and e being applied to the points a and b,respectively, the switching burst e causes the storage capacitor C to becharged to the ON potential e as the diodes D and D allow the capacitorto so readjust in the presence of the switching burst e With the cell ELin the ON condition, light feedback is then supplied, as shown in thefeedback path P in FIG. 1,

The diodes D, and D are 7 thus effecting the low impedance state of thephotoconductive resistor R in its low impedance state, thephotoconductive resistor R is lower than the resistance of the resistorR The control potential at points and thus tends to drift in thedirection of the ON potential or that at point g, thus maintainingstable operation in the ON state with the feedback path being providedto activate the photoconductive resistor R While in the embodiment shownin H6. 1 the photoconductive resistor R and the resistor R are shown asseparate circuit components with special input terminals, in practicalcircuits they may be combined with the other circuit elements. Becausesemiconductor diodes and electroluminescent cells are photoconductors,the photoconductor resistor R in this circuit can be combined witheither of the diode D or the electroluminescent capacitor EL. Similarly,the resistor R may be combined with the diode D the storage capacitor Cor the ferroelectric capacitor F FIG. 1 is only illustrative of one ofthe many possible circuit configurations that could be utilized with thesame basic concepts. Direct storage without the need for the storagecapacitor C could also be used. See Patent No. 3,018,412 issued .lanuary23, 1962 to I. A. Asars and assigned to the same assignee as the instantapplication.

Only one element of a display screen has been specifically describedherein. In order to provide a display screen a plurality of elements maybe arranged in a suitable matrix to visually display incoming electricalinformation.

Although the present invention has been described with a certain degreeof particularity, it should be understood that the present disclosurehas been made only by way of example and that numerous changes in thedetails of construction and the combination and arrangement of parts maybe resorted to Without departing from the scope and the spirit of thepresent invention.

1 claim as my invention:

1. In image display apparatus operative with information signals havingtwo levels the combination of, storage means operative to store saidinformation signals at either of said levels,electroluminescent-ferroelectric circuit means operative to provide alight output having either of two stable states in response to saidstored information signals, storage adjust means operative with saidstorage means to selectively allow said storage means to adjust inresponse to the then present level of said information signals, andphotoconductor stabilizing means responsive to said light output tomaintain the then 1 resent stable state.

2. A bistable electrical-to-light transducer information signal havingtwo levels the combination of, storage means operative to storeelectrical information at either of two levels,electrolurninescent-ferroelectric circuit means operative to provide alight output having either of two stable states in response to saidstored information level, storage adjust means operative with saidstorage means to selectively allow said storage means to adjust inresponse to the then present level of said information, andphotoconductor stabilizing means responsive to said light output tomaintain the then present stable state.

3. in ima e display apparatus operative with information signals havingtwo levels the combination of, storage means operative to store saidinformation signals at either of said levels,electroluminescent-ferroelectric circuit means operative to provide alight output having either of two stable states in response to saidstored information signals, storage adjust means operative with saidstorage means to selectively allow said storage means to adjust inresponse to the then present level of said information signals,stabilizing means responsive to said light output to maintain the thenpresent stable state, said stabilizing means including a photoconductivedevice having an impedance proportional to the light applied thereto.

4. in image display apparatus operative with information signals havingtwo levels the combination of, storage means operatively connected toreceive said information signals and being operative to store saidinformation signals at either of said levels,electroluminescent-ferroelectric cncuit means operativery connected tosaid storage means and being operative to provide a light output havingeither of two stable states in response to said stored informationsignals, storage adjust means operatively connected to said storagemeans and being operative to selectively allow said storage means toadjust in response to the then present level of said informationsignals, and stabilizing means operatively connected to said circuitmeans and being responsive to said light output to maintain the thenpresent stable state, said stabilizing means including a photoconductiveelement having an impedance proportional to the amount of light appli dthereto.

5. In image display apparatus operative with information signals havingtwo levels the combination of, storage means operatively connected toreceive said information signals and being operative to store saidinformation signals at either of said levels,electroluminescent-ferroelectric circuit means including a ferroelectriccapacitor connected to an electroluminescent capacitor with a commoncontrol junction therebetween to which said stored information signalsare applied, said circuit means operatively connected to said storagemeans and being operative to provide alight output from saidelectroluminescent capacitor having either of two stable states inresponse to said stored information signals, storage adjust meansoperatively connected to said storage means to selectively allow saidstorage means to adjust in response to the then present level of saidinformation signals, and stabilizing means responsive to said lightoutput, said stabilizing means including a photoconductive device havingan impedance proportional to the amount of light applied thereto, withsaid device being connected to said control junction and beingresponsive to maintain the then present stable state.

No references cited.

1. IN IMAGE DISPLAY APPARATUS OPERATIVE WITH INFORMATION SIGNALS HAVINGTWO LEVELS THE COMVINATION OF, STORAGE MEANS OPERATIVE TO STORE SAIDINFORMATION SIGNALS AT EITHER OF SAID LEVELS,ELECTROLUMINESCENT-FERROELECTRIC CIRCUIT MEANS OPERATIVE TO PROVIDE ALIGHT OUTPUT HAVING EITHER OF TWO STABLE STATES IN RESPONSE TO SAIDSTORED INFORMATION SIGNALS, STORAGE ADJUST MEANS OPERTIVE WITH SAIDSTORAGE MEANS TO SELECTIVELY ALLOW SAID STORAGE MEANS TO ADJUST INRESPONSE TO THE THEN PRESENT LEVEL OF SAID INFORMATION SIGNALS, ANDPHOTOCONDUCTOR STABLIZING MEANS RESPONSIVE TO SAID LIGHT OUTPUT TOMAINTAIN THE THEN PRESENT STABLE STATE.